This is on a W124 with the floating single piston calipers.

The pedal went to the floor at one point and it was due to a bad line. I replaced that and was not able to get a firm pedal but the car did stop well enough. Every weekend for the past month I've been trying to fix it and bled the brakes a dozen times. I finally went around and clamped all the soft lines, with the proper tool, and found the problem was coming from the front left. Upon further inspection I can see the pad moving back and forth causing this problem. I assumed it was a bad caliper and bought another. It is still doing it with the new caliper and the pedal feels fine if I have a helper clamp that corner so its something messed up with the new caliper.

To clarify the pedal feels fine with the entire brake system hooked up and the one line clamped near the caliper. With the caliper off the car I can actually see the piston move out and then back into the bore. This sounds like air in the piston but after beelding gallons of fluid though it how can that be?